PH.D. SCHOOL IN PHYSICS (XXVIII CYCLE), “SAPIENZA” UNIVERSITY OF ROME

EFFECT OF HIGH PRESSURE ON THE STRUCTURAL AND

VIBRATIONAL PROPERTIES OF ORGANIC MOLECULAR CRYSTALS

Ph.D. Thesis Project Candidate: Francesco Capitani Supervisor: Prof. Paolo Postorino

SCIENTIFIC BACKGROUND Molecular crystals (MCs) have considerably different structural, electronic, optical and chemical properties

compared to conventional solids such as covalent or ionic crystals. This is due to the weak interactions

taking place between molecules, like Van der Waals, dipole – dipole or Hydrogen bonds, compared to the

relatively strong interactions between atoms in non-molecular solids. Properties of MCs are also

determined by the subtle interplay between intermolecular and intramolecular interactions. These

features, along with the almost infinite choice of molecules as crystal building blocks, have made MCs

appealing in various fields of the scientific research in the last twenty years [1]. In particular, a very active

field on this topic is that of crystal engineering (or supra-molecular chemistry), and this is quite

straightforward if one consider that the aim of crystal engineering is the design of a MC with the desired

physical and chemical properties given only the molecular structure [1]. Therefore, crystal engineering is

quite a matter of theoretical prediction of possible MC structures based mostly on the interplay between

two main features: close packing [2] and intermolecular interactions. The principle of close packing relies

mainly on geometrical considerations, in other words the most stable thermodynamic phase at ambient

conditions is that reduces voids in the crystal structure[2]. Although this has been verified for a large

number of compounds, the existence of very weak and similar kinds of intermolecular interactions allows a

lot of metastable phases the system can kinetically approach [1,3]. This considerations, together with the

fact that the great majority of MCs are formed by organic molecules (that means tens-hundreds of atoms

per unit cell), are responsible for a really complicated energy landscape of MCs already at ambient

conditions [1,3]. This is reflected by the high polymorphism exhibited by organic MCs [3-5]. Despite this

intricate framework, at ambient conditions theoretical models are reliable (when the atoms in the unit cell

are not more than 100) and experimental works capture well the above mentioned features [1,3].

However, when the system is not at ambient conditions, i.e. high pressure (HP) or low-high temperature,

theoretical prediction of MC structures is no more reliable [3,5]. Therefore, systematic experimental works

at extreme conditions are needed in order to have a better understanding of the physics of these systems

and to provide a more severe test for the developing of more accurate theoretical models.

PROJECT Among organic MCs, those formed by polycyclic aromatic hydrocarbons, i.e. molecules made up by fused

benzene rings, have recently attracted the attention of condensed matter physicists. This is because of the

first discover of a superconducting transition in aromatic MCs of picene (C22H14)[6], coronene (C24H12)

[7], phenanthrene (C14H10) [8] and dibenzopentacene (C30H18) [9], upon doping with alkali metals, alkali

earths or rare earths. This discover has led to the appearance of several scientific works, mostly theoretical

[10-12]. Although the discovery has prompted a huge effort in the scientific community, there is still a

remarkable lack of experimental works as a consequence of the complex synthesis of superconducting

samples which, even when successful, produces samples unstable to air exposure and with very low

superconducting fractions.

For this reason, and considered the importance of a systematic and detailed knowledge of the physical

properties of the undoped samples to gain insight on the superconducting phase, my first Ph.D. year has

been devoted to the study of the vibrational properties of aromatic MCs under HP. Pressure has been

chosen because having experimental data on a wide pressure range is very useful for the development of

theoretical models more reliable than those tested on data collected at ambient pressure. Moreover, since

in superconducting samples doping apparently induces both volume change and charge transfer, the use of

HP on pure samples may allow to disentangle these two effects by singling out the modifications of the

vibrational spectrum induced by volume compression only. In particular, my first study has been focused on

solid picene and has been reported in two publications [13,14]. Using Raman and Infrared Spectroscopy,

Diamond Anvil Cells for the generation of HP and with the help of theoretical calculations performed by the

group of Prof. L.Boeri (Graz University of Technology) has been possible to show that picene, despite a

∼20% compression of the unit cell, does not undergo phase transitions up to 8 GPa, with vibrational modes

displaying a smooth and uniform hardening with pressure[14]. Accordingly, the observed reduction of the

peak frequency of the vibrational modes in the doped samples has been ascribed mainly to a charge

transfer effect [14]. It is worth to notice that the performed theoretical calculations showed a very good

agreement with the measured data, making us confident about the used theoretical model. The good

performance of this experimental/theoretical approach has led us to perform a similar study on solid

phenanthrene, but at the moment data analysis and calculations are in progress.

For the next years, the aim of this Ph.D. project is to complete this spectroscopic study and extend it to

other aromatic MCs, moving the main target from superconductivity to the study of intermolecular

interactions in MCs under HP, following the motivations presented in the previous section (see Scientific

Background). To reach the goal, X-Ray Diffraction (XRD) will be the main tool to probe the structural

properties under HP, together with optical spectroscopy and theoretical calculations. The combination of

these techniques, will allow a detailed exploration of the HP region of the phase diagram of some aromatic

MCs. This means trying to understand how the fundamental interplay between close packing and

intermolecular interactions change as a function of pressure and if this lead to new structural phases. This

study, aimed to provide a better understanding of the fundamental physics inside MCs, can also be useful

in other fields, like applied physics, crystal engineering and chemistry. This is quite clear considering that

physical and chemical properties interesting for technological devices [15, 16] or for designing a MCs with a

specific task (like drug delivery[17] or pollutant sequestration[18]) depends on the fundamental

interactions between the molecules in the crystal.

At the beginning, the study will be focused on aromatic molecules belonging to the same class of picene,

i.e. molecules with fused benzene rings in a zig-zag fashion. Picene has 5 benzene rings, so phenanthrene

and chrysene, respectively 3 and 4 rings, are the first candidates. Limiting the study to a well-defined class

of molecules, with known molecular and crystal structure at ambient conditions [19], will allow to reduce

the degrees of freedom to control (usually very high in MCs) and, therefore, to make it more feasible

achieving the fixed goal. In second instance, the study could be extended to MCs with molecules formed by

the same number of benzene rings, but in a different configuration, or with molecules with the same

configuration but increasing the number of benzene rings.

EXPERIMENTAL The project will be based on the use of several experimental techniques, as well as a collaboration with

Prof. L. Boeri (Graz University of Technology) and Dr. Marc Hӧppner (Max Planck Institute, Stuttgart) for

theoretical calculations. A Raman micro-spectrometer for Raman spectroscopy measurements, as well as

Diamond Anvil Cell for high pressure, will be provided by the High Pressure Spectroscopy group (Prof. P.

Postorino) of the Physics Dept. of this University. XRD and Infrared spectroscopy measurements will be

carried out in international synchrotron facilities, like ESRF (Grenoble, France) or ELETTRA (Trieste, Italy). In

particular, the XRD measurements will be in collaboration with the group of Dr. L. Malavasi of the Physical

chemistry Dept. of the University of Pavia, as well as Infrared measurements will be possible thanks to the

well-established collaboration with the SISSI beamline (Prof. S. Lupi) of the ELETTRA synchrotron.

REFERENCES [1] G.R. Desiraju, J. Am. Chem. Soc. 135, 9952 (2013) – perspective article

[2] A. I. Kitaigorodsky, Molecular crystals and molecules, Academic press (1973)

[3] G. R. Desiraju, Nature Mater. 1, 77–79 (2002)

[4] J. Bernstein et al., Angew. Chem., Int. Ed. 38, 3440 (1999)

[5] S.L. Price, “Predicting crystal structures of organic compounds” Chem. Soc. Rev. (2014)

[6] R. Mitsuhashi et al., Nature 464, 76 (2010)

[7] Y. Kubozono et al., Phys. Chem. Chem. Phys. 13, 16476 (2011)

[8] X.F. Wang et al., Nature Comm. 2, 507 (2011)

[9] Xue, M. et al., Sci. Rep. 2, 389 (2012)

[10] A. Subedi et al., Phys. Rev. B 84, 020508 (2011)

[11] T. Kato et al., Phys. Rev. Lett. 107, 077001 (2011)

[12] T. Kambe, et al., Phys. Rev. B 86, 214507 (2012)

[13] B. Joseph et al., J. Phys.: Condens. Matter 24, 252203 (2012)

[14] F. Capitani et al., Phys. Rev. B 88, 144303 (2013)

[15] C. Dimitrakopoulos et al., Adv. Mater. 14, 99 (2002)

[16] A. Troisi et al., J. Phys. Chem. B 109, 1849 (2005)

[17] S.L. Price, Adv. Drug Delivery Rev. 56, 301 (2004)

[18] D. D’Alessandro, Angew.Chem., Int. Ed. 49, 6058 (2010)

[19] G.R. Desiraju et al., Acta Crystallogr. B 85, 473 (1989)